This invention relates to oxidizer assemblies and, in particular, to oxidizer assemblies for use in proton exchange membrane (“PEM”) fuel cell applications.
In copending application U.S. Ser. No. 10/894,993, filed Jul. 20, 2004, entitled OMS-2 Catalysts in PEM Fuel Cell Applications, there is disclosed an oxidizer assembly which utilizes an OMS (“octahedral molecular sieve”)-2 catalyst to oxidize the carbon monoxide in the fuel feed to a PEM fuel cell. As described therein, OMS-containing materials, such as synthetic todorokite (Mg2+0.98-1.35Mn3+1.89-1.94Mn4+4.38-4.54O124.47-4.55H2O) or cryptomelane (K-hollandite, KMn8O16nH2O), comprise manganese oxide octahedral compounds linked by edges and vertices and forming uniform tunnels therethrough. OMS-2 catalysts are manganese oxide octahedral molecular sieves possessing the 2×2 tunnel structure (as in the aforementioned cryptomelane).
The '993 application specifically describes transition metal cation doped OMS-2 catalysts which can be framework-substituted and tunnel-substituted molecular sieves which are referred to by the designations [M]-OMS-2 and [M-OMS-2], respectively, where M indicates tunnel or framework-substituted metal cation(s) other than manganese. Specifically disclosed in the application as preferable catalysts are Co-OMS-2, Cu-OMS-2 and Ag-OMS-2, with Ag-OMS-2 being most preferable.
The '993 application also describes the operation of the OMS-2 catalyst to cause selective oxidation of the carbon monoxide in the feed to a PEM fuel-cell as occurring chemically via a sorption-chemical oxidation process aided by the unique pore structure and active sites of the catalyst. In particular, the sorption-chemical oxidation process at low temperatures is described as a two stage process, a sorption stage and a chemical oxidation stage. As stated therein, during the sorption stage, carbon monoxide is selectively adsorbed on the metal active side of the M-OMS-2 (Ag-OMS-2) catalyst as follows:Ag*+CO→COad  (1)
This process then proceeds to the chemical oxidation stage in which carbon monoxide is chemically oxidized with oxygen typically present in the OMS-2 tunnel or provided with the fuel feed or reformate gas. Specifically, oxygen from the OMS-2 tunnel is released in the following reaction:O-OMS-2→OMS-2+½O2  (2)Subsequently, carbon monoxide is oxidized by reacting carbon monoxide with the released oxygen to produce carbon dioxide in the following reaction:COad+½O2→CO2+Ag*  (3)
Following the sorption-chemical oxidation reaction, the OMS-2 can be regenerated in situ by adding oxygen from a feed gas to produce an O-OMS-2 regenerative substrate. This reaction is as follows:OMS-2+½O2→O-OMS-2  (4)
The oxidizer assembly incorporating the OMS-2 catalyst is described in the '993 application as being capable of performing the oxidation and the regeneration processes simultaneously. In particular, an oxidizer assembly is disclosed in which parallel packed bed reactors each having an M-OMS-2 catalysts are operated so that one reactor is performing carbon monoxide oxidation of the PEM fuel cell feed gas, while the other reactor is having its M-OMS-2 catalyst being regenerated. While this type of oxidizer assembly is usable, a more compact and simpler oxidizer assembly is desired.
It is therefore an object of the present invention to provide an oxidizer assembly for oxidizing the carbon monoxide in a feed gas which is simple and compact in configuration.
It is a further object of the present invention to provide an oxidizer assembly of the above-mentioned type which is also capable of allowing in situ catalyst regeneration without interrupting the oxidation of carbon monoxide.